This invention relates to a process for the production of a liquid, bromine-containing alkoxylation product.
Polyurethane and polyisocyanurate foams have numerous and varied applications in the fields of civil engineering, interior decoration and insulation. The flame-resistance of the materials used in such applications is an important property. As legislative regulations relating to fire behavior become more strict, the search for materials which meet these stringent legislative standards has intensified.
Several methods for rendering polyurethane foams flame-resistant are known. In one known process non-combustible additives (such as antimony oxide) or halogenated and/or phosphorus-containing compounds (such as tris-(dibromopropyl)-phosphate or tris-(dichloropropyl)-phosphate, chlorinated biphenyls and halogenated hydrocarbons are added to the foam. However, such additives are not incorporated into the polymer framework and do not produce lasting and homogeneously-distributed flame-resistance. Furthermore, such additives generally have a plasticizing effect on the foam and thereby impair the foam's mechanical properties, particularly pressure-resistance and dimensional stability.
In another method for the production of flame-resistant polyurethane foams, halogenated and/or phosphorus-containing polyols are used. The halogen may be either aromatically or aliphatically bound. However, experience has shown that aromatically bound halogen, as described, for example in U.S. Pat. No. 4,128,532 does not provide good flame-protection, possibly due to the high dissociation energy of the halogen-carbon bond. Such compounds have a relatively high melting point.
French Pat. No. 1,350,425 discloses use of halogenated polyether polyols (produced by addition of epichlorohydrin to polyvalent monomeric alcohols containing at least two hydroxyl groups) in the production of polyurethanes. The cellular polyurethanes obtained by reacting organic polyisocyanates with such halogenated polyether polyols have satisfactory flame-resistant properties but their dimensional stability is only moderate. Furthermore, such polyether polyols are unstable when stored in the presence of amine compounds such as those typically used in the production of polyurethane foams. This instability may be attributed to the fact that hydrogen halide cleaves off very easily from the aliphatic halogen compounds (unlike the aromatic halogen compounds).
It is known that markedly improved flame protection may be achieved by using bromine rather than chlorine. A bromine-containing polyether obtained by alkoxylating dibromobutene diol is disclosed in U.S. Pat. No. 3,764,546. Dibromobutene diol has the advantage that it does not cleave off hydrogen halide as easily as the aliphatic halogen compounds disclosed in French Pat. No. 1,350,425. In the process disclosed in U.S. Pat. No. 3,764,546 the dibromobutene diol is introduced into a polyol and is subsequently alkoxylated by acid catalysis. This process suffers from several disadvantages. For example, the dibromobutene diol must be produced in pure form. Although it is possible to produce such a diol in pure form, the bromination of butynediol in a solvent, filtration, recrystallization and drying is relatively costly. Further, the dibromobutene diol produced in this manner has to be redissolved for alkoxylation. This makes the entire synthesis substantially more difficult. A further disadvantage is that not only the dibromobutene diol but also any remaining polyol mixture is reacted during alkoxylation. Consequently unreacted dibromobutene diol remains. This diol can be filtered off after the reaction, but the diol which remains in the product precipitates when blowing agents conventionally used in rigid foam formulations are added.
According to German Offenlegungsschrift No. 2,445,571, halogenated polyether polyols are obtained by reacting dibromobutene diol with epichlorohydrin. Good flame-protection can be achieved with these products. However, the relatively high viscosity of the products is disadvantageous during processing (particularly metering). The synthesis of these halogenated polyether-polyols is also very costly. In addition to the problems which arise during production of alkoxylation products of dibromobutene diol, a chlorohydrin must be converted into an oxirane (with the cleaving of HCl gas) which is opened by alkaline hydrolysis in order to obtain the desired hydroxyfunctional compound.
In another process (JP No. 78 115 797) for the production of bromine-containing polyols, butynediol is reacted with propylene oxide in the presence of an acid catalyst and then halogenated with bromine. This method is disadvantageous because only about 75 to 80% of the butynediol is reacted during alkoxylation of the butynediol at the desired minimal mol masses (i.e. 1 mol of butynediol reacted with from 1.5 to 2.5 mols of alkoxide). (See Examples 4, 5, 10 and 11 infra.) A very wide isomer spectrum is also obtained when an acid catalyst is employed. In contrast there are essentially only two isomers which are formed when an alkaline catalyst is used. (See Examples 4 and 5 infra). When free butynediol is halogenated to produce 2,3-dibromobutene diol by subsequent bromination, the 2,3-dibromobutene diol partially precipitates immediately (Example 10) or after the addition of a blowing agent. This precipitation problem in particular hinders the use of these products.
The incomplete reaction of butynediol with alkoxides can be avoided if an alkaline catalyst is used instead of an acid catalyst, as described in U.S. Pat. Nos. 3,366,557 and 3,292,191. However, the alkaline catalysts have a tendency to cause a higher stage of hydroxyalkylation and to catalyze the reverse reactions. Consequently, unreacted butynediol or too highly hydroxyalkylated butynediol is often found in the end product. The presence of unreacted butynediol may cause explosions under alkaline conditions due to the instability of the butynediol. Too highly hydroxyalkylated butynediol renders the product so impure that it is no longer usable in many fields. The use of amines as basic catalysts would not overcome these disadvantages because the relatively high temperatures which are required lead to an increased tendency of the butynediol to form by-products and to explosively decompose.
German Offenlegungsschrift No. 2,036,278 discloses use of basic ion exchanger resins as basic agents. These basic ion exchanger resins have substantially improved the selectivity of the reaction but do not eliminate the remaining disadvantages of a basic process such as reverse reaction and risk of decomposition.
German Offenlegungsschrift No. 2,241,156 discloses a process in which thiodialcohols with a low mol composition (particularly thiodiglycol) are used as catalysts. This process suffers from the disadvantage that compounds such as thiodiglycol tend to form readily volatile thioethers under the reaction conditions. Such thioethers give the product such as unpleasant smell that use of the product becomes questionable. Conventional means for eliminating smell, such as hydrogen peroxide treatment, are unsuccessful in this case so that the products which have been catalyzed in this manner have an unacceptable smell. Moreover, thiodiglycol is not an optimal catalyst. The alkoxylation product of 1 mol of butynediol with 2 mols of ethylene oxide still contains a relatively large quantity of the starting material butynediol (See Example 13 infra).
There are as yet no known processes for providing an odor-free alkoxylation product with a low mol composition and a low monomer content (low butyne-1,4-diol content) from butynediol and alkoxides. Moreover, there is no commercially useful synthesis of the corresponding brominated products.
The use of 2,3-dibromobutene-1,4-diol as the starting diol (U.S. Pat. No. 3,764,546) is uneconomical for the reasons discussed above. The method disclosed in Japanese No. 78 115 797 cannot be applied in the described manner because it does not produce neutral products. The bromination of alkoxylation products of butynediol produces a substantial quantity of hydrogen bromide in a secondary reaction which cannot be suppressed. This hydrogen bromide must be removed by neutralization and filtration. This seriously complicates the synthesis and renders it more expensive. Experience has also shown that such bromination products are not stable in storage. Thus, over a period of a few days to weeks a marked rise in the acid number (a fall of the pH value) is observed. This change in the pH value is caused by traces of weakly-bound bromide which is slowly cleaved off as hydrogen bromide during storage. This change in pH value makes it necessary to continually modify processing conditions.
It can be seen from the prior art that different methods are known for producing bromine-containing polyols. However, none of these methods is a straightforward synthesis for liquid bromine-containing products which are stable in storage and are low in 2,3-dibromobutene-1,4-diol.